In recent years, many motor vehicles have been designed in a manner that the drive shaft of the internal combustion engine can not be used to drive the cooling fan necessary for directing a convection air current through the radiator, which air current is needed, especially when driving at slow speed in hot weather. With the advent of front wheel drive vehicles having transverse engines and limited air intake openings, many motor vehicles are now being manufactured with one or more electrical motors driving fans for cooling the radiators. These fans may be operated only when the radiator coolant temperature or other operating parameters indicate a need for additional external cooling, as often determined by an onboard computer. Consequently, the cooling fan may be turned on only when the vehicle is stopped or operated in heavy traffic. These cooling fans may not be used for long periods, such as during the depths of the winter in certain locations. Even such diverse times as the fans are used, they must operate on demand. Otherwise, damage to the engine can result or the engine must be stopped immediately. Even though these fans are extremely critical, most automobile operators are unaware of their existence or their operation. Thus, they must be extremely reliable.
To operate the cooling fans as described above, it is standard practice to provide a driver circuit with some type of power switch or power relay, which driver circuit generally includes a printed circuit board having circuits for controlling the power switch in response to a demand signal from the vehicle. This demand signal can be a pulse width modulated, high frequency signal wherein the duty cycle determines the demand for one or more cooling fans to meet the cooling requirement of the engine. The power switch and associated printed circuit board containing the driver circuit for one or more motors are relatively small and must be inexpensive, since they are required by the millions in the automobile industry. These driver circuits must be monitored to prevent damage to the motor and the power switch itself by high currents, which currents can occur when the motor is stalled or shorted.
A substantial amount of effort has been devoted to controlling these motor driver circuits with miniaturized components so that these circuits and components can be mass produced at low cost and still assure that the relatively expensive motors are not damaged, irrespective of the load condition across the motors. The most successful of these prior miniaturized circuits is a control circuit for the motor driver circuit, which control circuit applies a low level testing current across the motor and through the driver circuit to determine the condition of the motor or electrical load. If the testing circuit senses a high current condition, the main power is not applied to the motor circuit. If there is no high current condition, then the main power switch is actuated to drive the motor at a speed, determined by the duty cycle of the applied power, generally this is full power. This prior circuit is quite successful, is low in cost and has proven to be useful for the intended purposes; however, this prior circuit does have some characteristics which add to its cost, even though the cost has heretofore been acceptable. One of the characteristics is that after the motor driver circuit has been initiated, a separate and distinct circuit must be employed for monitoring for high current flow through the motor. When this separate circuit detects a high current flow, the driver switch is deactivated. The need for a separate and distinct circuit used in combination with the control circuit increased the cost of the miniaturized unit. Further, a mechanical relay was often used. When the motor was turned on, there was an in-rush of current which could trip or actuate the current sensor. In some instances, the current sensor measured the voltage across the power switch and deactivated the driver circuit when this voltage exceeded a certain level indicating a high current flow. This type of current sensor of the prior device, as well as a current sensor utilizing a coil associated with a reed switch, must be calibrated so that it does not deactivate the motor driver during the in-rush of current at the start. This increases the set point for current protection. This conflict in current calibration has been somewhat overcome by the use of a current sensor in combination with a low level testing current. In this manner, the in-rush current levels are reduced during the testing of the motor circuit. Irrespective of the combination of various circuits, the concept of measuring current flow through the motor by various arrangements is not totally satisfactory. They usually deactivate the motor driver without providing an integrated restart feature. In a general summary, the prior devices involved a level of current sensed high enough to assure ability of starting the motor without tripping. Consequently, the current sensing capacity of the prior control circuits, during normal operation, was at the higher adjusted current level. This caused a delay in the sensing of high current flow which could cause damage to the motor driver circuit.
If the prior control circuits for the motor driver circuits of a motor vehicle fan system allows high current flow, the motor can be damaged before the sensing circuit is activated. This dilemma is not solved completely by combining a system using low level testing current. The motor or driver circuit can still be damaged when there is a direct short or stall in the motor. All these prior systems have involved some time delay drop out which, in some instances, is not fast enough to protect the motor or driver circuit.